In the production of a semiconductor device, microfabrication by lithography using a photoresist composition has been conventionally performed. According to the increase in integration degree and operation speed of an LSI in recent years, further microfabrication is demanded by the design rule of the pattern, and under the circumstances, the lithography technique associated with exposure to light, which is currently used as a general-purpose technique, is approaching the essential limit of resolution due to the wavelength of the light source. The light source for lithography used for forming a resist pattern is being decreased in wavelength from KrF excimer laser (248 nm) to ArF excimer laser (193 nm). However, various problems arise due to further microfabrication.
One of the major issues relates to the aspect ratio. An ArF resist is relatively low in etching resistance and thus is necessarily increased in aspect ratio, but it is difficult to increase the aspect ratio due to collapse of the resist pattern. As a method for forming a pattern with a high aspect ratio, a three-layer resist method or the like has been proposed. In the method, a material for forming a ground coat is coated on a substrate and formed into a film by heating to form an underlayer film, and an intermediate film containing an inorganic film, such as a silica film, is formed thereon. A photoresist film is then provided thereon, and a resist pattern is formed by an ordinary photolithography technique. The intermediate film is etched with the resist pattern as a mask to transfer the pattern thereto, and then the underlayer film is etched using oxygen plasma with the patterned intermediate film as a mask, thereby forming a pattern on the substrate.
A two-layer resist method has also been proposed, which is favorable owing to the less number of process steps as compared to the three-layer resist method. In the two-layer resist method, after forming an underlayer film in the same manner as in the three-layer resist method, a photoresist film containing a silicone-containing polymer is formed as an upper layer thereof, forming a resist pattern by an ordinary photolithography technique, and etching using oxygen plasma is performed with the resist pattern as a mask to transfer the resist pattern to the underlayer film. Thereafter, etching using a carbon fluoride series gas is performed with the resist pattern as a mask, thereby forming a pattern on the substrate (Non-patent Document 1).
As a material for forming the underlayer film for 193 nm, a copolymer of polyhydroxystyrene and an acrylate ester is being generally studied. Polyhydroxystyrene has significantly strong absorption at 193 nm and has solely a high value around 0.6 for the extinction coefficient (k). The k value can be controlled to around 0.25 by copolymerizing an acrylate ester, which has a k value of substantially 0.
However, the acrylate ester is low in etching resistance upon etching the substrate as compared to polyhydroxystyrene, and furthermore, it is necessary to copolymerize the acrylate ester at a high proportion for decreasing the k value, which brings about consequently decrease in resistance upon etching the substrate. The etching resistance influences not only the etching rate but also generation of surface roughness after etching, and thus increase of the surface roughness after etching faces severe problem by copolymerization of the acrylate ester.
A naphthalene ring is one of structures that have high transparency at 193 nm as compared to a benzene ring and high etching resistance, and an underlayer film having a naphthalene ring or an anthracene ring has been proposed (Patent Document 1). However, a naphthol-copolycondensed novolak resin and a polyvinylnaphthalene resin have a k value of from 0.3 to 0.4 and fail to achieve the target transparency of from 0.1 to 0.3, and thus the transparency thereof is necessarily further increased. An acenaphtylene polymer (Patent Documents 2 and 3) has a low refractive index (n) at a wavelength of 193 nm as compared to 248 nm and a high k value, both of which fail to achieve the target values. Furthermore, proposals have been made for an underlayer film obtained by adding an acrylic resin to a naphthol-copolycondensed novolak resin (Patent Document 4), an underlayer film containing a polymer compound obtained by copolymerizing indene and a compound having a hydroxyl group or an epoxy group and having a double bond (Patent Document 5), and an underlayer film containing a polymer compound obtained by copolymerizing a novolak resin with fluorenebisphenol (Patent Document 6), but the target value k of from 0.1 to 0.3 has not yet been achieved.
Furthermore, the material for an underlayer film also involves a problem with a sublimable component. There is such a severe problem that a sublimable component forms crystals on the surface of the upper plate upon baking, and the crystals drop onto the wafer to form defects. Due to the reason, a material that contains a less amount of a sublimable component is demanded. The conventional material uses a polymer, such as a novolak resin, owing to the demand of etching resistance, but contains a monomer and unreacted dimer and oligomer, which have sublimability, and therefore, an increased number of process steps are required for removing the sublimable component, which largely influences the production cost.
Accordingly, such a material for an underlayer film is demanded that has a high refractive index (n) and a low extinction coefficient (k), is transparent, has high etching resistance, and contains a considerably small amount of a sublimable component.    [Patent Document 1] JP-A-2002-14474    [Patent Document 2] JP-A-2001-40293    [Patent Document 3] JP-A-2002-214777    [Patent Document 4] JP-A-2005-156816    [Patent Document 5] JP-A-2006-53543    [Patent Document 6] JP-A-2007-17867    [Non-patent Document 1] PROCEEDINGS of SPIE, vol. 4345 (2001), 50